How to bring back the mainframe
It is not called a mainframe, it is called the Outliner. This took me a while to figure out what you meant, but that's fine.
Look very closely at the top right corner inside where the outliner should be. See that gray arrow pointing down? Click it or drag it down. A bar should slide down. Go to the left side of that bar, and there is another dropdown. Click it, and from the window types select Data-&gt;Outliner.
Click the arrow to unhide the header and switch to Outliner.